Body mass index, or BMI, is a calculation that compares your weight relative to your height. The index isn’t perfect. A bodybuilder’s BMI may indicate that she’s overweight, for example, but her extra weight is muscle rather than fat. For pretty much everyone else, however, BMI is a reasonable measure of your fitness level, demonstrating whether you’re overweight, underweight or just right. If your BMI isn’t where you want it, exercising can help you lose weight, gain muscle, or both.

Calculating BMI

You can find online calculators to figure your BMI, or you can convert your height and weight into kilograms and meters and tally the results yourself. Just remember that 1 pound equals 0.454 kilograms and 1 inch equals 0.0254 meters. To determine your BMI, divide your weight in kilograms by your height in meters squared. For example, if you’re 5 feet 5 and weigh 150 pounds, you’re 1.65 meters tall and weigh 68.1 kilograms. Divide 68.1 by 2.72 (that’s 1.65 squared) for a BMI of 25. The normal BMI range is 18.5 to 24.9. Anything less than 18.5 is underweight. A range of 25 to 29.9 is overweight, while anything more than 30 is considered obese.

Aerobic Exercises

If you’re only a touch overweight, adjusting your diet may be all you need to edge your BMI into the normal range. But the most efficient way to lose weight and keep it off is to combine a healthy diet with aerobic exercise. To lose 5 percent or more of your body weight, for example, you must do approximately five hours of moderate-intensity exercise per week. Try to burn at least 500 extra calories each day, which translates into a loss of 1 pound per week. Aerobic activities can include brisk walking, which burns 314 calories per hour for a 160-pound woman, plus lap swimming (423 calories), running at 5 mph (606 calories), low-impact aerobics (365 calories) or jumping rope (861 calories).

Weight-Training Exercises

Weight training can offer a double benefit. You’ll work off some calories while you’re exercising, and you’ll raise your metabolic rate if you build more muscle mass, because muscles use more energy than fat, even when you’re at rest. Resistance training itself burns about 365 calories per hour, if you weigh 160 pounds. You’ll burn more calories by circuit training, which involves performing a series of consecutive exercises with no rest between activities. Circuit training burns roughly 600 calories per hour for a 160-pound woman. Try compound exercises -- activities that move multiple joints and target more than one muscle group at a time -- such as deadlifts, rows, standing shoulder presses, barbell squats, bench presses and dumbbell lunges.